History of Astronomy: Don’t memorize

datesWho were the scientists?What did they contribute to what we

know today?When (approximate) did they make

this contribution? What is the significance of that time

frame?

Ancient History-Greeks (400B.C.-A.D. 150)

Aristotle (384-322 B.C.)Determined that the earth is round. He

observed shadows cast on the Moon during lunar eclipses.

Before Christopher Columbus!

Greeks-Two modelsHeliocentric Model - Aristarchus(312-

230 B.C.) = first Greek to believe this modelFigured out the size of and distances to the

moon and sun.Used that to determine the sun as the

center of the universeEarth and other planets revolve around the

sun.Not accepted by most Greeks

Ancient History - Greeks

Eratosthenes (276-195 B.C.)Determined circumference of the earth

using geometry and trigonometry.

Hipparchus (190-120 B.C.)Determined the location of 850 “fixed”

stars and the length of the year.

Greeks-Two modelsGeocentric Model

Earth is the center of the universe.Celestial sphere = transparent bubble

that contained all the stars, planets, etc.Fixed stars = all the heavenly bodies

that remain stationary on the celestial sphere

Wanderers = 7 bodies that move within the fixed (background) stars.Included the sun, moon, Mercury, Venus, Mars,

Jupiter, and Saturn

Greeks- Accepted Model

Claudius Ptolemy- Proposed the Ptolemaic System (A.D. ~127-145)Circular OrbitsGeocentric modelUnchallenged for 13 centuries because it

predicted the motion of the planets.Explained Retrograde motion-planets

move eastward across the night sky, but sometimes will stop and go backward before going eastward again.

Ptolemaic System

Recent HistoryNicolaus Copernicus (1473-1543)

Proposed Earth as a planetSupported heliocentric modelExplained most motions by a rotating

Earth.Religious/Social implications = goes

against the Roman Catholic ChurchThink Inquisition of 1600’s! Acceptance

means to be fined or ostracized.

Tycho Brahe (1546-1601)Famous for detailed and precise

measurements of the heavensExtensive amounts of very precise

dataConclusions? None published, the

Catholic Church reigns supreme, also a believer of Ptolemaic System. (Trying to prove it with his research.)

Galileo Galilei (1564-1642)

1609 constructed a telescope(first to use for astronomy, not the inventor.)

Discovered 4 satellites, moons, of JupiterData that supported Copernicus

Planets are roundVenus has phasesMoon’s surface has mountain, craters,

etc.Sun has sunspots, and rotates.

Johannes Kepler (1571-1630)

Assistant to BraheKept/Stole/Inherited data?Using Brahe’s data, discovered the

three laws of planetary motion.

Kepler’s Laws of Planetary MotionLaw Number 1

The orbit of a planet around the sun is an ellipse with the sun's center of mass at one focus.

Law #1-SummaryAll orbits are ovals!We draw circles with one focus.

Ellipses have two foci instead of one. A.k.a. a mathematically perfect oval.Circles are a special kind of ellipse.

So, all circles are ellipses, But all ellipses are not circles.

In orbits, the sun is at one focus.

Sun (focus) Path of planet(ellipse)

QuickTime™ and aDV - NTSC decompressor

are needed to see this picture.

Kepler’s Laws of Planetary MotionLaw Number 2

A line joining a planet/comet and the Sun sweeps out equal areas in equal intervals of time.

Another way of saying this is that the planet moves FASTER in its orbit the closer it is to the sun.Perihelion = Point in orbit where planet is

closest to the sun, therefore, traveling fasterAphelion = Point in orbit where planet is

farthest from the sun, therefore, traveling slower

QuickTime™ and aDV - NTSC decompressor

are needed to see this picture.

Great! So?Why would the planet move faster as

it got closer?Many factors to consider. . .Can partially explain it by the

Universal Law of Gravitation.

Sir Isaac Newton(1642-1727) Universal Law of Gravitation

All things have a force of gravity. Dependant upon mass and distance.

The greater the mass the more gravity. The closer two objects, the more gravity.

INVERSE RELATIONSHIP! Difference between mass and weight.

Mass is the same everywhere. Weight is a measure of the force of gravity. Why don’t the Astronauts drift away from the moon?

2nd law Tie in: Force of gravity is greatest when the planet is closest to the sun.

Gravity in a nutshellMass matters!

We fall to the earth, not our neighbor, because Earth is so much more massive.

Astronauts feel lighter on the moon because it is less massive and has less gravity.

Distance matters!We fall to the earth, not the sun, because Earth

is much closer.Sun is still massive enough to keep Earth, and

other planets in orbit.How does it relate to Kepler’s Laws?

When the planet is nearing perihelion, it will feel a greater force of gravity from the sun. It will go faster!

Gravity according to Einstein (1879-1955)

Einstein has given us a frame of reference for understanding Newton’s law of gravity.

Bent space model - Anything with mass “bends” space. More massive objects will stay put and bend

the space around them. Less massive objects will “sink” in toward the

more massive ones.Will investigate this in lab later this week.

Einstein’s Gravity

Kepler’s Laws of Planetary MotionLaw Number 3

The period of a planet increases rapidly with the radius of its orbit. (T2=d3)

Period=Time it takes for the planet to make one revolution around the sun.

Law #3-Summary Don’t memorize the

formula! (We’ll save that for your physics class.)

Logically-it takes longer to travel a larger distance. Think about the outside

lane on a race track vs. the inside lane.

PlanetDistance from sun

Period

Mercury 0.38 AU 88 days

Neptune 30 AU 60,266 days(165

years)